Canon 300mm F.4 IS plus 1.4 extender...best aperture.

Was just wondering what is the best aperture to shoot birds with. I used it for the first time today.............is F.8 actually F.8 or is it actually F.10. Does the camera do the convertion for you or do you have to do the maths yourself...........sor​ry about my dumbassness! Without the 1.4 extender I find F.8 the best aperture for birds ( providing theres enough light )....would this change once add the extender. Any help much appreciated! Thank you

I'm not at all into bird photography, but regarding any sort of shooting, I'm guessing the sweet spot of the 300 f/4 IS with 1.4x lies between F/8 and F/11, without a whole lot of difference between it. Of course it also depends on whether or not you have a background you want to blur out. In that case I'd sacrifice some sharpness to get a shallower DoF. The same would count for a higher ISO in poor light.

Check photozone.de
Are you using FF or Crop? Crop bodies start to had diffraction from f/8.0.

I usually test my lenses at various f stops when I first get the lens. Looking at my results the 300mm f4 IS is sharpest at f8 and with a Canon 1.4 extender attached it's sharpest at f10.

I like mine at f11, it's very sharp.